The present invention relates to a two-stage vacuum pump apparatus for evacuating, to a high level of vacuum, a space in which a work has to be conducted under such a high level of vacuum. The invention is also concerned with a method of operating such a two-stage vacuum pump apparatus.
One of the important requisites for a vacuum pump apparatus is to prevent a system to be evacuated from being contaminated by oil. A roots pump is known to be capable of satisfying such a demand to a high extent. In general, however, the roots pump can develop only a low compression ratio, so that it is combined, as proposed in Japanese Unexamined Utility Model Publication No. 5789/1984, with an oil-sealed rotary vacuum pump in order to produce a medium level of vacuum on the order of 10.sup.-2 to 10.sup.-4 Torr. and higher levels of vacuum. The compression ratio developed by a roots pump is as low as 2 to 5 in the atmospheric pressure region (about 760 Torr.) and does not exceed 20 to 70 even in the medium pressure region of 10.sup.-2 to 10.sup.-4 Torr. It is, therefore, required to connect the outlet side of the roots pump with an oil-sealed rotary vacuum pump which is capable of operating to attain a higher compression ratio. In such a case, a pressure difference on the order of 1 to 10.sup.-3 Torr. is developed across the oil-sealed rotary vacuum pump. When the oil-sealed rotary vacuum pump operates in such a condition, vigorous vaporization of the sealing oil takes place in this pump, causing a risk that the evacuated system is contaminated by the vaporized oil transferred through the roots pump.
U.S. patent application Ser. No. 701,199 proposes to use a screw pump under atmospheric back pressure (760 Torr.). The use of the screw pump, however, is not preferred because this type of pump consumes a large electric power and requires a large amount of torque at the time of start up, thus necessitating the use of a driving motor which is capable of producing a torque much greater than the torque required during steady evacuating operation.